Potentiometers are electric devices that are used in both analog and digital circuits, most often as voltage dividers where a constant voltage output is needed at an output terminal. Potentiometers can also be used in rheostat mode, where only two terminals of the 3-terminal potentiometer are used and a variable resistor is created. Unlike traditional analog potentiometers which are adjusted mechanically, digital potentiometers can be adjusted using software serial interfaces. A digital potentiometer presents advantages over an analog potentiometer because of its higher resolution, its ability to store more than one wiper position, and its cost-effectiveness in implementing.
A digital potentiometer has three terminals: two input terminals and a third terminal referred to as the wiper. In rheostat mode, only the wiper and one input terminal are used. The other input terminal is either shorted by directly connecting with the wiper, as shown in prior configurations in FIGS. 4 and 5, or left floating with no connection. Operating in this mode allows for the digital potentiometer to be configured as a variable resistor. Depending on which input terminal is shorted with the wiper or left floating, the value of this resistance is the sum of the resistances between input terminal A and the wiper, Ra, or input terminal B and the wiper, Rb, as shown in FIGS. 4 and 5.
A problem with current architecture for digital potentiometers is that they have a high tolerance when operating in rheostat mode. This tolerance is typically on the order of 30% and is problematic for circuit design when specific resistance values are needed. The current architecture of digital potentiometers is further restrictive because the resistance value of Ra, the resistance between input terminal A and the wiper, cannot be calibrated independently of Rb, the resistance between input terminal B and the wiper, as shown in FIGS. 4 and 5. FIG. 1 shows a previous configuration of a digital potentiometer, where the string array S5 is common to the string array between terminal A and the wiper, S3, and the string array between terminal B and the wiper, S4. Therefore, neither Ra nor Rb, the resulting resistances between the input terminals and the wiper when a switch within each string array is closed, can be adjusted without changing the value of the other corresponding resistance.
Despite the presence of both Ra and Rb, only one variable resistor can be created from a digital potentiometer in rheostat mode. The present invention overcomes this limitation as well.
Furthermore, in the prior art, the number of strings present in the circuit was expressly limited to 2n+1 (where n is a natural number), the calculation of which is always an odd number, a reflection of the presence of only one string connected to the wiper. The present invention is able to overcome this limitation, with the number of strings present in the integrated circuit equal to 2n which is reflective of the connection of two independent strings to the wiper terminal.